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Most poly is 6mm. It is surprisingly strong. A pressure washer will not damage it. Not that you have to worry about snow, but a single layer of 6mm was enough to collapse my high tunnel from snow, without tearing the plastic first. It pulled five 4x4 posts set in concrete, at each end, about two feet at the top. The force required was enormous. My other greenhouse held, by my guess, over 2,000 pounds of snow and ice on the plastic with no damage.

If you use double-layer poly, you'll need an inflation blower. Use outside air to inflate. Also, if you have roll-up sides, use a single layer for that part, not a double, because it can't be inflated. Without inflation, the layers of plastic rub together and turn cloudy very quickly.

The newer plastic has an "infrared block" quality to it. It is opaque, not clear. It is a lot better about not getting too hot in full sun. Clear plastic will make a solar cooker in the late spring and summer.

Folks, I have decided to build a greenhouse using old house windows that people are throwing in dumpsters around here. Thanks for all your suggestions - I looked at them all, considered carefully, and decided that the best thing that will work for me is a glass greenhouse, customized to my lot, and built of free stuff that would otherwise go into the landfill. I will post a photo here when the 14 by 25 foot greenhouse is done.

Those old windows were made to be mounted vertically. They are great for that purpose in your greenhouse. However, they were never meant to be a roof. You will have to overcome the issue of water pooling on them, and you won't want to be underneath them the first time a decent-sized hail stone hits, or it will maim you for life.

Hail isn't an issue here in Atlanta (well, except once in a decade or so), but tree limbs are the big issue. I am thinking of putting crosswise 2x2s across the roof to try to reduce the impact of falling limbs, but I am sure I will need to replace panes now and then.

I'm trying to think how to keep water from pooling along the downhill edges of the window panes. Maybe some clear silicone on those places would do the trick. If water sits anywhere, it will rot the wood.

I'm trying to think how to keep water from pooling along the downhill edges of the window panes. Maybe some clear silicone on those places would do the trick. If water sits anywhere, it will rot the wood.